Tape recording and reproducing machines generally have tape transported between a supply reel and a take-up reel. The reels are motor driven and the motors are controlled by servos to maintain a predetermined tension on the tape. The tape is moved at relatively slow speed during recording and reproducing and can be moved very rapidly during a shuttle operation. Tape speeds of approximately 600 inches per second during shuttle operation are known in broadcast quality video tape recording and reproducing apparatus.
During high speed shuttle operations, the tape is often driven by the reels which are in turn controlled by reel servo systems.
In the event of a power failure, the driving motors eventually stop, largely as a result of frictional retarding forces. The retarding forces may be generated within each motor and at places contacted by the moving tape. Because of these forces, the tape may be subjected to substantial and varying tensile loadings. These loadings are undesirable because they may physically damage the tape or cause pressure erasure of magnetic signals recorded on the tape. Additionally, the inertia of the takeup reel may be less than that of the supply reel because of differences in the quantity of tape stored on the reels at the time of a power failure, whereby the supply reel rotates at a speed relative to that of the take-up reel which discharges tape at a rate greater than that at which the take-up reel is capable of receiving tape. As a result, tape may be spilled during stopping instead of being taken up on the take-up reel. The possibilities of tape spillage from the supply reel and accompanying tape damage become especially likely during shuttle operations because of the very high speeds involved.
Tape reels of existing tape recording and reproducing machines are generally equipped with mechanical brakes which are often released by a solenoid in the event of a power failure. Typically, the mechanical brakes operate differentially in that they exert greater braking torque upon the reel when supplying tape than upon the reel when receiving tape. Dynamic brakes also have been employed in tape transports wherein the motor current resulting from the back EMF of the motor effects braking of the motor upon the occurrence of a failure.
While such brakes have generally been satisfactory for use in longitudinal audio and quadruplex video recording and reproducing machines, they often provide less than satisfactory performance in helical wrap recording and reproducing machines. The helical wrap machines have tension problems which are aggravated, for example, by the use of a large heavy duty tape reel together with a small light weight plastic reel that many users find convenient. In addition, the difference in the inertias of the two reels due to continuously varying differences in the tape packs on the reels at any point in time, can cause corresponding continuously varying differences in the dynamic braking requirements. It follows that present typical continuously operating mechanical and/or dynamic braking systems fail to provide means for controlling the application of braking forces during the entire deceleration of the motor until it comes to a stop.